1. Field of the Invention
The present invention relates generally to printing systems. More particularly, the present invention relates to a carrier system for providing positive control over a printable medium being processed by a printing system, to prevent damage to the printable medium.
2. State of the Art
As processing speeds of printing systems continue to increase, their handling of the printable medium being processed becomes increasingly more difficult. For example, with respect to printing systems which are fed a web-like printable medium, desired processing speeds are approaching, and even exceeding, rates of three thousand feet per minute.
The processing of a web-like printable medium includes, for example, the cutting of the web-like printable medium along its feed direction into two or more continuous webs, or ribbons. Each of the ribbons is then separately processed to create sheet-like signatures by cutting each ribbon at regular intervals in a direction transverse to the feed direction. Each resulting signature incudes a leading edge and a trailing edge relative to the feed direction. Processing of the web-like printable medium can additionally include, for example, folding of the ribbon prior to its being cut into individual signatures.
To avoid damage to signatures produced by cutting the ribbon, it has been conventional to pin the ribbon to the cutting cylinder. This operation effectively constrains the leading edge of the ribbon to prevent its damage. For example, the ribbon is pinned onto cutting cylinders of a folding device used to fold and then cut the ribbon into signatures. However, this technique requires that the pinned leading edge of the ribbon be removed from each resultant signature in a post processing operation. Such a technique thus wastes the printable medium and involves additional processing. Accordingly, more recent developments in the handling of web-like printable mediums have been directed to the use of so-called pinless folders.
Pinless folders eliminate pinning of the ribbon to the cutting cylinder prior to transversely cutting the ribbon to separate the trailing edge of a downstream signature from the leading edge of the ribbon. However, pinless folders suffer an attendant loss in control over the ribbon's leading edge after the cutting process. This loss in control can result in downstream damage to the signatures. For example, the signatures can become bent at the corners of the leading edge. The use of pinless folders therefore limits the speed with which a printable medium can be processed. Accordingly, attempts to increase the processing speed of a printing system without damaging the signatures has resulted in efforts to regain control over the leading edge of the ribbon, without requiring a pinning of the leading edge to the cutting cylinder.
Two solutions used to address the foregoing problem are: (1) tacking of the ribbon's leading edge to the cutting cylinder via static electricity; and (2) corrugation of the ribbon as it is fed to the cutting cylinder. However, these solutions suffer attendant drawbacks. For example, the first solution involves electrically charging the ribbon so that static electricity can be used to hold the ribbon's leading edge to the cutting cylinder. However, where the ribbon has been folded prior to being cut into signatures, this electrical charging of the ribbon creates problems in post press processing where the folded signatures must be reopened. That is, the electrically charged, folded signatures resist opening during post press processing.
The second solution involves introducing corrugations to the ribbon to stiffen the ribbon for transport to the next area of constraint, such as a downstream signature deceleration device. However, the mechanical devices used to corrugate the ribbon are high wear devices, which are sensitive to adjust. As such, these devices are difficult to maintain, and require a high level of operator intervention.
In addition to the foregoing drawbacks, the use of techniques such as tacking and/or corrugation to control a ribbon's leading edge in a pinless folder is relatively ineffective at higher web speeds; for example, web speeds on the order of three thousand feet per minute or greater. In addition, these techniques become ineffective as the weight of the ribbons and/or signatures is reduced. As such these techniques have been deemed unreliable, even when used in combination.
Accordingly, it would be desirable to positively control a printable medium during its processing in a printing system, without suffering the drawbacks associated with conventional printing techniques.